When carrying an object by a crane, a rope extends from a driving motor which drives a trolley, and the object is suspended on the rope. During such a carriage, the object suspended on the rope is oscillated, with the result that the work efficiency and the working safety are aggravated.
That is, the oscillations of the object carried may collide with a worker, thereby causing an accident. Further, when stopping the trolley for unloading of the object, the object may oscillate continuously. Therefore, the operator either should wait until the oscillations stop, or another worker should hold the object to stop the oscillations of the object.
In a prior art, the following method has been proposed for eliminating the oscillations of the object in a crane operation. In Japan, there has been proposed a method such that, during the carrying, an open circuit is used to adjust the acceleration and deceleration patterns, and, after the stop of the crane, a closed circuit is used to remove the oscillations. In this method, an open circuit is used during a carrying, and therefore, when an external disturbance exists such as the occurrence of a collision and the blowing of wind, a perfect oscillation control cannot be assured.
Besides, there is in Japan a purge estimation theory in which the oscillations of the object are not controlled during the carrying, but the oscillations are damped upon stopping of the crane. In this method also, the oscillations are controlled during the carrying, and therefore, the oscillation control is not perfect as in the case of the former.
In Finland, there is a method in which a computer varies the control sampling time in accordance with the variation of the length of the rope, thereby forming an algorithm for damping the oscillations of the object. According to this method, a closed circuit is applied during the carrying, with the result that the oscillation control is carried out in an acceptable manner. However, after the carrying, a position controller is not employed, with the result that a precise position control cannot be realized.